Because of a recent tendency to pursue size-reduction, speed-up and high-pressure in hydraulic machinery, such equipments including a cylinder block of a motor or of a hydraulic pump or motor, an under body, and a variable speed shaft bearing tend to be used under harsh conditions. For example, the volume of incidental facilities, such as a catalyst, tend to take more space in the motor chamber of axial piston hydraulic pumps and motors, used for power shovels and other such machines in construction machinery, in order to comply with the emission regulations that get stricter every year. As a result, the size of hydraulic pumps has to be reduced. However, to demand the same level of discharge rate as before from the hydraulic pumps with reduced-size will require their use under higher pressure of more than 450 bars and higher speed of more than 3,000 rpm.
In such circumstances, a sliding member that is subject to high pressure, such as the cylinder block of the hydraulic pump, is required to maintain stable sliding capacity even under the aforementioned harsh conditions. In order to obtain such stable sliding capacity under the harsh conditions, the combination of a steel piston and a cylinder block using copper alloy as its sliding material is essential. A composite material in which a layer of copper alloy is solidly deposited on steel has also been developed for such sliding member.
However, since the sliding member subjected to high pressure, such as the cylinder block of the hydraulic pump described above, is also required of high seizing resistance, lead bronze that contains about 10 mass % of lead (Pb) is mainly used for the aforementioned copper alloy. The lead contained in the copper alloy has small affinity for both steel and copper alloy, and is an indispensable element instrumental in obtaining high seizing resistance as well as high abrasion resistance. In the case where the lead content in the aforementioned lead bronze is lowered, its seizing resistance ability is also reduced, thus making it inadequate as a material for the sliding member of hydraulic equipments that tend to be used under harsh conditions.
On the other hand, from the standpoint of environmental conservation, the efforts to reduce lead content in copper alloy to make it low lead or lead-free are gaining ground. In recent years, there is a move to prohibit or decrease the use of environmental load substances, such as lead and cadmium, in various industrial products. Thus, for example, in order to comply with the European environmental regulations, lead content in copper-tin alloy has to be reduced to 4 mass % or less.
In response to such a move, various efforts have been taken toward the reduction of lead content in copper alloy for the sliding member, and a variety of sliding materials to achieve the reduction of lead content, with higher sliding capability, have been suggested. For example, a copper-based or iron-based sliding material with more than 1 variety of bismuth (Bi)-based intermetallic compounds are dispersedly precipitated therein is disclosed in JP h11-293305. This sliding material is claimed to have achieved lead-free and higher seizing resistance by means of solid lubrication effect deriving from bismuth-based intermetallic compounds.
In JP 2002-285262, a copper-based multi-layered sliding material including a steel plate and sintered copper alloy bonded thereto is disclosed. The composition of the above sintered copper alloy is tin (Sn):1.5 to 15 mass %, Bi: 1.5 to 15 mass %, solid lubricant: 1.5 to 20% by volume, with remains Cu, while the volume ratio of Bi and solid lubricant is 0.5 to 2.0%. An invention described in JP 2002-285262 also offers a lead-free copper-based multilayered sliding material that has sliding capability higher than or equal to the lead bronze sintered alloy.
In WO 2007/126006 A1, a copper alloy for sliding material which is either solid solution hardened or solid solution strengthened by both solid solution and compound generation, containing 0.05 to 1.5 mass % sulfur (S) with an intermetallic formation of Cu2S compounds or Cu2S compounds+ZnS compounds, is disclosed. This copper alloy for sliding material, containing at least either 0.1 mass % or more and 11.0 mass % or less Pb, or 0.1 mass % or below 5.4 mass % Bi (these do not undergo solid solution in a copper matrix), where they are found alone or as PbS compound or Bi2S3 compound. The copper alloy described in this publication claimed to have enabled both abrasion resistance and seizing resistance at high level and achieved long life as a material for the sliding member even under harsh conditions by means of inclusion of appropriate amount of S.
Further, in JP 2007-297675, a lead-free casting copper alloy that contains S, Bi, Fe (and/or Ni) and has sulfide dispersed therein is disclosed. This invention disclosed in JP 2007-297675 relates to the casting lead-free copper alloy for a material for water faucet clasps, wetted stopper and the like. This invention realizes superior machinability and pressure resistance without containing lead that can cause harm to human body, while preventing an occurrence of a cavity inside a casting as sulfide is effectively dispersed in the copper matrix, by means of including iron (Fe) and nickel (Ni) in the copper alloy along with Bi, the free-machinable element.
However, the prior arts described in JP h11-293305, JP 2002-285262 and WO 2007/126006 A1 use a large amount of sulfide or compounds of low-melting-point metals, such as bismuth, as additives to the extent that it results in crystallization in the copper (Cu) base so that such crystallization serves as solid lubricant. The problem with this approach is that it is not viable from the standpoint of production cost and casting. For example, bismuth costs as much as 5 times more than copper metal, and can result in cost escalation if bismuth be added by 10 mass %. Another problem is that a copper alloy that features solid lubrication of crystallized compounds or sulfide can only realize the seizing resistance at half the level of lead bronze.
On the other hand, as explained above, the prior art described in JP 2007-297675 aims at producing lead-free casting copper alloy for the use as a material for water faucet clasps and wetted stopper, and thus is not sufficient from the standpoint of achieving both abrasion resistance and seizing resistance at high level. The focus of the invention according to JP 2007-297675, as explained in the paragraph 0018 for example, is on the improvement in machinability by means of sulfide formation while inhibiting an occurrence of a cavity, by taking into account that the melting point of the generated copper sulfide is made lower than the primary crystallization point (the first coagulation point) of copper due to the addition of Bi, Fe or Ni. In the first place, the invention according to JP 2007-297675 is intended for lead-free casting copper alloy for the material for water faucet clasps, wetted stopper and the like. Accordingly, for example, it intends to include Sn in the range of 3 to 4%, and thus gives no consideration to abrasion resistance and seizing resistance.